1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic method, such as a copying machine, a laser printer, or a facsimile, and more particularly, to measurement of an amount of adhered toner and control of an image density.
2. Description of the Related Art
In a method for controlling a density of an image formed by an image forming apparatus using an electrophotographic method, a toner image to be used for density detection is formed on a photosensitive drum or an intermediate transfer belt, and an image forming condition is feedback-controlled based on a density detected from the toner image.
The image forming condition includes a charging bias of a photosensitive drum bearing a charged toner, an amount of exposure light for controlling a density of the charged toner borne by the photosensitive drum, and a transfer voltage for transferring a toner image borne by the photosensitive drum or the intermediate transfer belt onto a recording material such as paper. If the image forming condition is determined, a density of the toner image transferred onto the recording material is determined.
In recent years, the height of the toner image borne by the photosensitive drum or the intermediate transfer belt has been able to be detected with high precision. A method for detecting a density of the toner image from the detected height of the toner image has been proposed. The density of the toner image changes depending on an amount of toner borne per unit area (an amount of adhered toner).
More specifically, if the amount of adhered toner is increased to form a toner image having a high density, the height of the toner image is also increased. If the amount of adhered toner is decreased to form a toner image having a low density, the height of the toner image is also decreased. Therefore, the density of the toner image can be detected by detecting the height of the toner image.
Japanese Patent Application Laid-Open No. 4-156479 discusses irradiating a toner image for density detection borne by a photosensitive drum or an intermediate transfer belt with light, and measuring the height of the toner image from a light receiving position on a line sensor for receiving light reflected from the toner image. The measured height of the toner image is converted into a density of the toner image by referring to data representing a correspondence relationship between a height and a density, which is previously stored.
However, in such an image forming apparatus, if a charge amount of toner in a developing unit changes due to an environmental variation such as temperature and humidity, deterioration of a developer that is a mixture of toner and magnetic carrier, or the like, a density of a toner image to be formed cannot be controlled with high precision because a correspondence relationship between the height of the toner image borne by the photosensitive drum or the intermediate transfer belt and a density of an image output from the image forming apparatus changes.
A case where a toner image having a predetermined height corresponding to a predetermined density is formed while an image forming condition is determined using a conventional method will be described below.
In a toner image developed using toner having a small charge amount, a density of the toner forming the toner image becomes high because a repulsive force exerted between its toner particles becomes low. Therefore, a toner image having the toner having a small charge amount borne thereon to a predetermined height is output as an image having a density higher than a predetermined density.
In a toner image developed using toner having a large charge amount, a density of the toner forming the toner image becomes low because a repulsive force exerted between its toner particles becomes high. Therefore, a toner image having the toner having a large charge amount borne thereon to a predetermined height is output as an image having a density lower than a predetermined density.